Here's another Isolated Transformer, connected as an Autotransformer - in this case, the connections are Series Subtractive - making it a voltage "Bucking" transformer.The schematic "Boost Transformer" is very similar, except the connections are made in Series Additive format.BTW: The "Buck" transformer is a mirrored image of the "Boost", the inputs and outputs are reversed.

FYI: The "PHI" symbol on each schematic represents positions of phase polarity [direction of magnetic flow in core as to direction of currents in windings]. That's the key issue of how and why these Autotransformers [connected as Autotransformers] can do the Buck / Boost thingee [technically speaking ]

>The "phi" symbol on each schematic represents positions of phase polarity [direction of magnetic flow in core as to direction of currents in windings].I would like to see the ø symbol added to the appropriate poles on the single phase transformer diagram you previously posted.